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‫So in the previous sections we made heavy use of inheritance through our actors.

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‫Now what is inheritance?

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‫Inheritance is when a child class automatically has all of the functionality of its parent.

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‫We say essentially that the child is a parent.

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‫So where did we use that previously?

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‫Well, for example, in our moving platforms, we had an actor that was a moving platform actor, and

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‫then we had different children of that actor that gave us the spinning, rotating platforms that up

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‫and down platforms, the side to side pushers.

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‫Those were all children.

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‫They all were movers.

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‫You could say the moving pusher is a mover.

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‫We could say that the rotating baton is a mover.

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‫So there is an alternative to using inheritance, which is a bit all or nothing.

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‫And this is composition which is a bit more pick and choose.

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‫You get to say that Class A has an instance of Class B, it can choose to use this functionality, but

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‫it doesn't have to.

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‫It's not tied in to being class B, it can just choose to take the bits it wants.

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‫So we say that Class A has a class B, so let's have a look at some examples of inheritance versus composition

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‫using something quite natural.

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‫I think talking about inheritance in the animal kingdom, we might have a class an actor say that is

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‫an animal actor and it has a method on it, which is to live its lives.

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‫When you call that method now, it makes sense to have things like a mammal that inherits from this

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‫because it also needs to live.

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‫So it needs all of that functionality that the animal has, but it adds some more functionality, such

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‫as making milk.

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‫We might also have a reptile that has the functionality, the added functionality of laying eggs.

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‫Now, it's fair to say that a mammal is an animal and a reptile is an animal as well.

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‫Now we could go onwards and say, okay, we've got a cow.

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‫Well, a cow clearly is a mammal.

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‫It needs to make milk.

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‫And by extension, it also needs to live because it's also an animal.

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‫A snake is a reptile and it is an animal, but it is not a mammal.

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‫So that makes sense, too.

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‫But what about a platypus?

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‫A platypus suddenly needs to both be a mammal and a reptile because it needs the functionality of laying

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‫eggs and it needs the functionality of making milk and it needs to be an animal.

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‫So it gets a little bit confusing here with inheritance.

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‫And inheritance can put us in a straightjacket and make these situations really hard to resolve, which

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‫is why composition is what comes to the rescue.

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‫Here is the has a relationship.

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‫So instead of trying to build this taxonomy of classes with animals and mammals, etc., what we do

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‫instead is we say we've got a milk making component and that component has the functionality of making

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‫milk.

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‫So the cow would have one of these milk making components and it gets its functionality.

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‫From there, the snake can have its egg laying component and it gets its egg laying functionality from

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‫there.

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‫And the platypus can have both components.

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‫It doesn't have to pick and choose one component.

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‫It can have all of them, and it gets to share the functionality just fine.

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‫So let's take the example that we're going to be using here, where we've got a mover, we've got something

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‫that wants to move.

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‫We might have started setting it up as an inheritance and is a relationship.

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‫So we've got a mover that inherits from this static mesh actor, kind of like we've done in previous

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‫sections, and then from the mover, we might inherit a moving door or a moving wall.

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‫But what happens if we wanted to do a moving light?

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‫How could that be done?

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‫We can't inherit from mover because that would make it a static mesh actor and we want to use a light

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‫actor instead as the parent for moving light.

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‫How could we resolve this problem?

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‫Using has a relationships instead pause a video, have a think about it, get out a piece of paper and

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‫jot down some of your thoughts.

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‫Okay.

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‫Welcome back.

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‫So this is the way I would resolve the problem so we could have our moving door and moving well.

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‫Both inherit from static.

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‫Actor That's fine.

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‫And the moving light can inherit from the light.

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‫Actor But now the difference is that we include this has a relationship in here as well.

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‫So the moving door, the moving wall and the moving light all can have a mover component on the actor.

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‫And this is where actor components come in really handy.

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‫And we're going to put this into practice in the next lecture, so I'll see you there.